1. Field of the Invention
The present invention relates to a coated paper used in printing and, particularly, to a new coated paper for use in printing which hardly generates any fluting in web-offset printing (in the Japanese printing industry, this may be referred to as “hijiwa”) which has been frequently generated during a process of drying after printing in web-offset printing. In addition, it also includes the manufacturing method of the coated paper. This is also very useful when used in rotogravure printing or flexographic printing from the standpoint that it will not cause so much out-of-register, i.e. mis-registration.
2. Description of the Related Art
First of all, an explanation will be given on the fluting in web-offset printing. The trend toward less man-power and higher speed in the printing industry in recent years is changing printing process from sheet-fed (flat sheet) offset printing to offset rotary printing (hereinafter referred to as “web-offset printing”). Not only high-speed printing and simultaneous-double-sided printing but also saving labor in its back-end process can be carried out by the web-offset printing. The productivity of the web-offset printing is significantly higher than that of the sheet-fed (flat sheet) offset printing in view of, such as, labor saving of its following process.
However, since a hot air drying process is conducted immediately after its printing process in case of web-offset printing, there are several quality defects that are not produced in sheet-fed offset printing. Among them, a problem that is known as most significant and difficult to solve is fluting in web-offset printing. Hitherto, the fluting in web-offset printing has been considered as a problem peculiar to web-offset printing, and is a phenomenon in which stripe-shaped wrinkles have been generated along the machine direction of the paper after a Web-offset printing operation is applied, which is apt to occur in a coated paper that is required good printing finish. In a worse case, the printed material would become waved, like a waved galvanized sheet iron, so that its substantial commercial value will be greatly lowered. Thus, a coated paper for printing which will not generate such fluting or wrinkles in web-offset printing has been strongly demanded for a long time. However, such a paper has not been provided to the market as or yet.
Now, several study reports have been issued on the aforementioned fluting in web-offset printing and they maybe roughly classified into the following two types:
One is based on the thought of “tension wrinkles.” In this theory, it is considered that fluting in web-offset printing is formed by wrinkles which is initially generated by the tension added to the paper in web-offset printing and is then fixed by offset ink.
As to the other, it is considered that wrinkles are generated by the difference in the thermal shrinkage between the imaged area and the non-imaged area during the drying process in the web-offset printing operation (Takeshi Yamazaki/Pulp and Paper Research Conference Proceedings of JAPAN TAPPI: Vol. 49, P110-113/1982.)
One of the methods proposed as concrete means to suppress such a phenomenon as disclosed in publication of Japanese unexamined patent application No. 186700/1983. In this method, such fluting in web-offset printing is considered to be prevented by keeping the freeness of pulp used in a paper web within a specific scope and by controlling the air permeability simultaneously within a specific scope.
However, at the time of manufacturing coated paper for use in web-offset printing, since the products are made through a series of processes such as the preparation of paper stock, paper-making, coating, press finishing with a calender and winding, it is not possible to obtain products in satisfactory quality merely by adjusting the pulp freeness or air permeability of the base paper. Any product that avoids the fluting in web-offset printing has not been made as of yet.
Further, according to publication of unexamined Japanese patent application No. 291496/1997, it is proposed that the fluting in web-offset printing can be either solved or alleviated by specifying the web moisture and the internal bond strength of a base paper. However, if the internal bond strength is lowered, this will require lowering of the moisture of a coated paper in view of countermeasure for blister resistance, which is considered as another problem of the coated paper for web-offset printing. As a result, there is a fear of causing a problem so called “fold-cracking trouble” which is a phenomenon that the surface of the coated paper for web-off set-printing is cracked in a subsequent bending process. Any improvement effect on solving the fluting in web-offset printing has not been made satisfactorily in accordance with the conventional method.
We, the inventors of the present invention, have sought for the factor generating the fluting in web-offset printing which is an important problem in quality and to be solved with regard to the coated paper for web-offset printing as mentioned above. And we have repeated careful studies so as to solve the problem. Consequently, we created the present invention, in which the fluting in web-offset printing can be prevented in advance by using a paper having small thermal shrinkage force in the cross direction (the CD direction).
Namely, since the fluting in web-offset printing has been generated mainly in coated paper having low basis weight (about the basis weight of 60 g/m2 and below), the countermeasures therefor are intended for such coated paper having low basis weight. However, since the fluting in web-offset printing is also seen in coated papers having rather high basis weight of greater than 60 g/m2 through the observation of the inventors, they have taken these facts into consideration and endeavored to obtain original coated paper that would not generate the fluting in web-offset printing.
Needless to say, the coated paper according to the invention will show significant effects in solving the fluting in web-offset printing, and besides “the mis-registration” which is easily caused by thermal drying can be effectively suppressed if it is utilized as paper for printing used in printing machines equipped with drying units, such as gravure printing machines and flexographic printing machines.